Preparation of mesophasic polyborazylene mesophasic polyborazylene and use as a BN precursor

ABSTRACT

The present invention relates to: 
     a novel process for the preparation of mesophasic polyborazylene. Said process is of particular value in that it affords a quality product from an appropriate polyborazylene, rapidly (virtually instantaneously), reproducibly and with a good yield. Furthermore, said process is easy to carry out. Said process comprises the preparation of polyborazylene by the polycondensation of borazine in a closed reactor and the addition, to said polyborazylene obtained by polycondensation, of a solvent selected from aromatic solvents, borazine solvents and mixtures thereof, 
     mesophasic polyborazylene which is novel in that it is in the presence of a particular solvent and/or by virtue of its quality, 
     the use of said polyborazylene of the invention, and/or prepared according to the invention, as a boron nitride precursor.

This application is the national phase of PCT/FR98/01403, filed Jul. 1,1998, now WO99/01389.

The present invention relates to:

a novel process for the preparation of mesophasic polyborazylene. Saidprocess is of particular value in that it affords a quality product froman appropriate polyborazylene, rapidly (virtually instantaneously),reproducibly and with a good yield. Furthermore, said process is easy tocarry out,

mesophasic polyborazylene which is novel in that it is in the presenceof a particular solvent and/or by virtue of its quality,

the use of said polyborazylene of the invention, and/or preparedaccording to the invention, as a boron nitride precursor.

Since the midseventies, the appearance of new technological stakes inmaterials sciences has rekindled interest in organometallic polymers,and numerous developments have appeared. Thus there have been reports onthe synthesis of siliconcontaining polymers, of the SiC and SiC/Si₃N₄type, capable of being shaped and of being used as precursors toceramics. These discoveries, combined with increasing practical needsfor molding compounds and for new materials for hightemperatureapplications, have encouraged new research into organometallic andinorganic polymers.

Boron nitride is the basis of commercial ceramics. It can easily beobtained in powder form by the pyrolysis of relatively inexpensivereactants. Unfortunately, it is very difficult to obtain said boronnitride in the form of fibers or films from powders. The use of polymersbased on boron and nitrogen seemed to be a very attractive way of doingthis. Thus polymers of this type, obtained by the polycondensation ofborazine, have already been used. Borazine or “inorganic benzene”, whichis easy to prepare, is in fact an appropriate precursor (being areactive planar molecule capable of generating planar polycondensedmolecules consisting of borazine rings). Said borazine has the followingchemical formula:

and is characterized by a B/N atomic ratio of 1.

In a first method, said borazine was polymerized in bulk, with regulardegassing (the dihydrogen is evacuated as it is formed), to generate apolymer, polyborazylene, which can be deposited especially in the formof a film. Said polyborazylene, its preparation by this method and itsuses as a boron nitride precursor have been described in particular by:

FAZEN P. J., BECK J. S., LYNCH A. T., REMSEN E. E., SNEDDON L. G., in“Thermally Induced Borazine Dehydropolymerization Reactions, Synthesisand Ceramic Conversion Reactions of a New Highyield Polymeric Precursorto Boron Nitride”, Chem. Mater., 1990, 2, 96-97;

FAZEN P. J., REMSEN E. E., BECK J. S., CARROLL P. J., McGHIE A. R.,SNEDDON L. G., in “Synthesis, Properties and Ceramic ConversionReactions of Polyborazylene. A Highyield Polymeric Precursor to BoronNitride”, Chem. Mater., 1995, 7, 1942-1956.

The specific use of said polyborazylene in the form of a film has beendescribed in particular by:

CHAN V. Z-H., ROTHMAN J. B., PALLADINO P., SNEDDON L. G., COMPOSTO R.J., in “Characterization of Boron Nitride Thin Films Prepared from aPolymer Precursor”, J. Mater. Res., 1996, 11(2), 373-380;

SNEDDON L. G., BECK J. S., FAZEN P. J., in “Direct Thermal Synthesis inthe Absence of Catalyst, and Ceramic Applications of Poly(borazylenes)”,patent U.S. Pat. No. 5,502,142.

In a second method, said borazine was polymerized in bulk with thepressure being allowed to increase in the reactor. Economy et al. showedthat, under such conditions, it was possible to obtain oligomers with avery low “graphitization” temperature (1500° C. instead of 2500° C. foran ordinary boron nitride) (graphitization being a process ofthree-dimensional organization of the sheets of BN hexagons, analogousto the organization of the graphite crystal).

To obtain, under these conditions, a polymer capable of developing amesophase, Economy et al. either stored the resulting oligomer forseveral weeks at a temperature of between 0 and 5° C., or baked saidoligomer at 100° C. under a pressure of 20 MPa. (Observations bypolarized light microscopy show that the application of a heat treatmentup to 100° C. has the effect of disordering the material. However, afterreturn to room temperature, Economy et al. observe a return to ananisotropic state, although this reversibility is not total. In fact,X-ray diffraction shows a peak corresponding to the (002) planes,broadened after treatment at 100° C.)

Both of the above methods, which therefore afforded mesophasicpolyborazylene, have been described respectively by:

KIM D., ECONOMY J., in “Occurrence of Liquid Crystallinity in a BorazinePolymer”, Chem. Mater., 1994, 6, 395-400 (storage at low temperature);

and by:

COFER C.G., KIM D., ECONOMY J., in “Formation of an Ordered BoronNitride Matrix for Fiber Reinforced Composites”, Ceram. Trans., 1994,46, 189-197 (baking under pressure).

However, said methods are difficult to reproduce and relativelyexpensive to carry out and, in any case, generate polyborazinemesophases of mediocre quality with only a very low yield.

Now, those skilled in the art are not unaware of how valuable it wouldbe to have easy and quantitative access to a sufficiently fluidmesophasic polymer which already contained broad anisotropic domains,foreshadowing the sheet structure of the desired ceramic, for use as aprecursor to boron nitride, especially hexagonal boron nitride (BN-hex),with a view to applications as a matrix and interphase, or even fibers,etc.

Faced with this technical problem of providing such a boron nitrideprecursor—mesophasic polyborazylene—the Applicant has developed a novelprocess for the preparation of said precursor. Said process makes itpossible to obtain said precursor under very advantageous conditions andin novel forms.

According to its first subject, the invention therefore relates to anovel method of preparing a mesophase which is a boron nitrideprecursor, said method affording the desired product (virtually)instantaneously and quantitatively and with a high yield. Said methodconsists essentially in adding a solvent to a polyborazylene. It has theadvantage of adjusting the viscosity while developing the liquid crystalstructure. This facilitates control over the rheology when shapingmaterials, such as an interfacial film for composite materials.

More precisely, the process proposed according to the invention is aprocess for the preparation of mesophasic polyborazylene which comprisestwo essential steps. In the first of said steps, polyborazylene isprepared by the polycondensation of borazine in a closed reactor. Thissynthesis of polyborazylene, which has been known per se since at least1959 (D. T. Haworth, L. F. Hohnstedt, J.A.C.S., 1959, vol. 82, 3860), iscarried out by the technique of the prior art referred to above. In thesecond of said steps, as a characteristic feature, the desired mesophaseis obtained virtually instantaneously by the addition, to saidpolyborazylene obtained by polycondensation in a closed reactor, of anappropriate additive (called a solvent) selected from aromatic solvents,borazine solvents and mixtures thereof.

Totally surprisingly, the Applicant has found that a polyborazyleneprepared in a closed reactor (i.e. in particular under the pressure ofthe dihydrogen released) leads instantaneously, under the effect of anappropriate solvent, to the formation of a mesophase, reproducibly andwith high yields.

Within the framework of the process of the invention, the simpleintervention of a suitable solvent is advantageously substituted for thetechniques of the prior art involving slow ageing at low temperature orbaking under pressure, which afford a mesophasic polyborazylene of lowquality, nonreproducibly and with only a low yield. Such a substitutionis advantageous by virtue of its ease of use, its high yield, itsreproducibility and the quality of the product obtained.

The solvent used, which dissolves the polyborazylene or is itselfdissolved in said polyborazylene, favors the organization of thepolycondensed molecules. Its use, in larger or smaller quantity (precisedetails on this point are given below in the present text), affords themesophase of greater or lesser fluidity.

Whatever the amount of solvent added, notably for the purpose ofadjusting the viscosity of the mesophase obtained or of removing saidsolvent from said mesophase, it is possible, within the framework of theprocess of the invention, to make provision for a third step involvingtreatment of said mesophase in order to remove all or part of saidsolvent therefrom. Those skilled in the art will know how to apply theappropriate techniques to achieve this end. Evaporation techniques,especially under vacuum, may be mentioned without implying a limitation.

Returning to the first step of the process of the invention—thepoly-condensation of borazine in a closed reactor—conventionally, saidpolycondensation is generally carried out at a temperature of between 50and 120° C., advantageously at a temperature of between 60 and 80° C. Itis obviously carried out under an anhydrous inert atmosphere. Inparticular, it can be carried out under a nitrogen or argon atmosphere.Conventionally, the initial pressure inside the reactor is atmosphericpressure. Within the framework of the invention, the process has alsobeen carried out under controlled initial inert gas pressures aboveatmospheric pressure, especially of between 10.10⁵ and 200.10⁵ Pa. Thusthe first step of the claimed process, namely the polycondensation, isgenerally carried out under an initial inert gas pressure of betweenatmospheric pressure and 200.10₅ Pa. It is not excluded from theframework of the invention to carry out said first step under a total orpartial pressure of dihydrogen (said dihydrogen constituting achemically inert gas in the present case).

For information, and without in any way implying a limitation, theApplicant is in a position to indicate here that the polyborazyleneobtained at the end of the first polycondensation step carried out in aclosed reactor generally has the following characteristics:$\begin{matrix}{{\overset{\_}{M}}_{n} \approx {600{–1400}}} & ( {{number}\text{-}{average}\quad {molecular}\quad {weight}} ) \\{{\overset{\_}{M}}_{w} \approx {1600{–5700}}} & ( {{w{eight}}\text{-}{average}\quad {molecular}\quad {weight}} ) \\{I_{p} \approx {2{–6}}} & ( {{polymolecularity}\quad {index}} )\end{matrix}$

which are determined by size exclusion chromatography (SEC) with apolystyrene standard. (It may be pointed out more precisely that themolecular weight analyses were performed using a Waters 510 apparatusequipped with a Waters 410 differential refractometric detector. Thecalculation of the different molecular weights is based on a calibrationobtained from monodisperse polystyrene standards under the followingconditions: column: TSK GMHXL, porosity: 1500 to 10⁷ Å; eluent: THF,flow rate: 1 ml/min, duration: 15 min.)

As a characteristic feature within the framework of the process of theinvention, said polyborazylene obtained at the end of the firstpolycondensation step in a closed reactor is brought into contact withan appropriate additive called a solvent. Said solvent is generallyadded to said polyborazylene, cooled to room temperature.

Said solvent is selected from:

aromatic solvents such as benzene, toluene, ortho-, meta- andparaxylenes, etc.

borazine solvents such as borazine and derivatives thereof Saidderivatives include especially Nalkyl and/or Balkylborazines (such asN-trimethylborazine and N-triisopropylborazine), the term alkylgenerally corresponding to linear or branched (C₁₋C₈)-alkyl groupsinsofar as the derivatives are liquid under the conditions applied. Saidderivatives are not substituted by reactive functional groups,

mixtures of said solvents.

In one advantageous variant, the solvent used is selected from borazine,benzene, toluene and xylenes.

Irrespective of its nature, said solvent is quite obviously used dry andwith an appropriate degree of purity.

Likewise, it is quite obviously used in a reasonable and appropriateamount. Said solvent is generally added in a solvent/polyborazyleneweight ratio below 10 (generally greater than or equal to 0.1) andadvantageously of between 0.2 and 5. In fact, said solvent is suitablyadded:

in a minimal amount in order to achieve the expected effect, namelyorientation of the mesogenic molecules and adjustment of the viscosityor fluidity of the mixture;

in a reasonable amount in order to avoid destroying the mesophase whenthe solution obtained is too dilute.

It has already been indicated that said solvent can be added in acertain amount during the second step of the process of the inventionand totally or only partially removed during a third step.

It is advisable to adjust the amount of solvent added to the intendedfinal use of the mesophase prepared.

The Applicant thus recommends:

adding said solvent in a solvent/polyborazylene weight ratio greaterthan 1, especially for the preparation of a mesophasic polyborazylenewhich is a precursor to interphases (fiber/matrix interphases in thestructure of a composite material) based on boron nitride;

adding said solvent in a solvent/polyborazylene weight ratio equal to orless than 1, especially for the preparation of a mesophasicpolyborazylene which is a precursor to fibers, matrices or materialsbased on boron nitride.

The value of the process of the invention will not have escaped thoseskilled in the art.

As already specified, said process affords a novel mesophasicpolyborazylene, which constitutes the second subject of the presentinvention.

Admittedly, according to the prior art, Economy et al. preparedmesophasic polyborazylene, but said mesophasic polyborazylene was not inthe presence of a solvent of the type used in the process of theinvention, nor was it in a very “pure” form.

The second subject of the present invention thus relates to:

mesophasic polyborazylene in the presence of a solvent selected fromaromatic and borazine solvents and mixtures thereof. All the detailsregarding the exact nature of said solvent and the amounts used,provided with reference to the process of the invention described above,can be repeated here. It is recalled that said polyborazylene of theinvention is in solution in said solvent or, conversely, that saidsolvent is in solution in said polyborazylene,

mesophasic polyborazylene in the presence or absence of such a solvent,characterized in light microscopy by a high degree of anisotropy. As acharacteristic feature, the mesophasic polyborazylene of the inventionhas a degree of anisotropy greater than 50%, advantageously greater than80% and capable of reaching about 100% in light microscopy. TheApplicant claims said mesophasic polyborazylene per se, which has neverbeen obtained according to the prior art. It has described above aprocess for the preparation of said product, based on the action of aspecific solvent on an appropriate polyborazylene.

The value of such a product was recalled in the introduction to thepresent text. It constitutes a preferred precursor to boron nitride,especially hexagonal boron nitride. Thus the third subject of thepresent invention relates to the use of a polyborazylene of theinvention having the characteristics listed above (the presence of aparticular solvent and/or a high degree of anisotropy in lightmicroscopy), and/or the use of a polyborazylene prepared by the processof the invention (comprising mainly the addition of a particular solventto a polyborazylene obtained by the polycondensation of borazine in aclosed reactor), as a precursor to boron nitride, especially hexagonalboron nitride.

Those skilled in the art are perfectly familiar with the heattreatment(s) to be carried out in order to obtain the appropriateceramization.

The boron nitride obtained from the mesophasic polyborazylene of theinvention is generally a boron nitride of the hexagonal type. It isconceivable that a nonhexagonal boron nitride might be obtainabledirectly from the mesophasic polyborazylene of the invention, but it ismost certainly obtainable by conversion, using methods familiar to thoseskilled in the art, of the hexagonal boron nitride obtained directlyfrom the mesophasic polyborazylene of the invention.

It is specified here, without in any way implying a limitation, that themesophasic polyborazylene of the invention (and/or prepared according tothe invention) can generally be used for the following purposes inparticular:

the production of coatings based on boron nitride;

the production of materials, especially fibers, based on boron nitride;and, more particularly, in the context of the preparation of compositematerials:

the production of fibers and/or interphases on the surface of fibersand/or matrices.

The mesophasic polyborazylene of the invention is in fact suitable forthe preparation of any type of product based on boron nitride,especially boron nitride of the hexagonal type. It is particularlysuitable, by virtue of its film-forming properties, for the productionof interphases and/or coatings. These are characterized by an excellenthomogeneity.

Those skilled in the art are perfectly familiar with the heat treatmentsto be carried out in order to obtain the appropriate ceramization.

The invention is now illustrated by the attached Figures and theExamples below.

FIG. 1 shows a light microscope negative between crossed nicols of amesophase of the invention obtained in the presence of a borazinesolvent (wave plate) according to the protocol of Example 1;

FIG. 2 shows a light microscope negative between crossed nicols of apolyborazylene aged according to the prior art (wave plate), moreprecisely according to the protocol of Comparative Example 1′;

FIG. 3 shows a light microscope negative between crossed nicols of amesophase of the invention obtained in the presence of a borazinesolvent (wave plate) according to the protocol of Example 2;

FIG. 4 shows a light microscope negative between crossed nicols of amesophase of the invention obtained in the presence of an aromaticsolvent (wave plate) according to the protocol of Example 3;

FIG. 5 shows bright field images of a freshly prepared mesophase of theinvention (according to Example 1) including, as an insert, the electrondiffraction spectrum (high resolution transmission electron microscopy);

FIG. 6 shows a dark field image and lattice fringe images (insert) of amesophase of the invention (prepared according to Example 1) from whichthe solvent has been at least partially removed and which has beenstored in air (high resolution transmission electron microscopy);

FIG. 7 shows an energy loss spectrum of a freshly prepared mesophase ofthe invention (according to Example 1) from which the solvent has beenat least partially removed;

FIG. 8 shows a low energy loss spectrum of a freshly prepared mesophaseof the invention (according to Example 1);

FIG. 9 shows a scanning electron microscope negative of a carbon bundlecoated with a film obtained from a mesophase of the invention (mesophaseprepared according to Example 2);

FIG. 10 shows a lattice fringe image (high resolution transmissionelectron microscope negative) of the texture of a boron nitride fiberobtained from a mesophase of the invention (mesophase prepared accordingto Example 2).

Comments on said FIGS. can be found in the Examples section below.However, the following statements will be made straightaway withreference to FIGS. 1 to 4 (light microscopy) and FIGS. 5 and 6 (electronmicroscopy).

The characterizations of the mesophases of the invention by lightmicroscopy are effected on an Olympus microscope equipped with a heatingstage (Leitz) under a stream of inert gas (argon). The mesophase isdeposited directly on the quartz sample holder, allowing observation intransmitted, polarized and analyzed light. A wave plate of λ=530 nm isused. The negatives are produced on Kodacolor Gold 100 ASA films.

The preparations for electron microscopy were made up in a glove boxunder nitrogen with a greatly reduced humidity level. The polyborazylenein borazine is simply deposited on copper grids coated with a perforatedcarbon film. The preparations are then rapidly placed under a low vacuumin the prepumping chamber before being introduced into the microscopeunder an ionic vacuum (fresh preparations). Some of these preparations,stored in air without special precautions, were observed again severaldays later. The preparations have solidified, contain bubbles and arestable under the beam.

The observations are made using a Philips CM30 ST apparatus aligned to300 keV in the low dose mode. The energy losses (determined by theElectron Energy Loss Spectroscopy (EELS) technique) were measured usinga Gatan 666-3K spectrometer and with the microscope aligned to 100 keVin the image mode so as to increase the energy resolution.

EXAMPLE 1 Synthesis of the Polyborazylene

The dehydrogenation/condensation reaction is carried out in a stainlesssteel autoclave of volume V=12 ml. The borazine charge (8.5 ml; 7.4 g;0.092 mol) placed in said autoclave under an inert atmosphere (P_(N) ₂o=10⁵ Pa) corresponds to about threequarters of its volume. Saidautoclave is then placed in an enclosure where the temperature ismaintained at θ=70° C. for 48 h. Under these conditions, the borazinepolycondenses to generate a low molecular polymer. The pressure insidethe autoclave, due mainly to the release of dihydrogen, is measuredafter return to room temperature. This pressure is 360.10⁵ Pa. Theweight of polyborazylene recovered is 6 g (yield=80%).

Analysis of said polyborazylene by ¹¹B NMR in the decoupled mode revealsa broad peak centered at δ=+30 ppm, comprising shoulders characteristicof borazine rings. Said analysis is performed using a Brüicker DPX 400spectrometer operating at 128 MHz, with tubes of 10 mm diameter.

Preparation of the Mesophase

The mesophase is obtained by mixing an amount m=110 mg of polyborazyleneprepared in this way with an amount m=130 mg of borazine. The solutionobtained is fluid and homogeneous. It has a milky appearance andpossesses all the characteristics of a liquid crystal in lightmicroscopy.

Characterization of said Mesophase

By Light Microscopy

Observation by polarized light microscopy of the mesophase obtained withborazine shows the existence of broad yellow or blue isoclinic domainsorientated at 45° to the polarizer or the analyzer (FIG. 1).

If polarized light passes through a birefringent crystalline medium, therays then propagate in two orthogonal directions with differentvelocities according to the indices of this medium (birefringence). Inthe analyzer, these two beams recombine to create an interference colorcharacteristic of the phase shift between the two beams. In the presentcase, if the drop of polyborazylene diluted in the solvent wereisotropic, it would behave like all liquids (or isotropic media). Therewould be complete extinction between polarizer and analyzer. The liquidwould appear black. On the other hand, if the molecules in this liquidpossess long range or liquid crystal orientation ordering, as is thecase, the light behaves as in a crystal and the liquid assumescharacteristic interference colors. When the birefringence is weak, awave plate which imposes a known phase shift equal to one times thewavelength is used in the optical path. The addition or subtraction ofthe phase shifts causes the orangeyellow and blue colors to appear inthe negatives, proving the existence of the mesophase.

The negative of FIG. 1 shows that the addition of borazine does indeedresult in the formation of liquid crystals. Furthermore, when the stageis rotated, a characteristic rolling extinction (curvature of theextinction contours) is observed, together with the existence of fixedsingularities which provide evidence of the presence of disclinacions(rotation defects typical of liquid crystals). The addition of a smallamount of solvent therefore allows the polymer to reorganizeinstantaneously, so the molecules containing planar borazine rings stackup and align themselves with long range ordering visible in the lightmicroscope.

By Transmission Electron Microscopy

The mesophase is simply deposited on copper grids coated with aperforated carbon film. The preparations are then rapidly placed under alow vacuum in the prepumping chamber before being introduced into themicroscope under an ionic vacuum.

The diffraction pattern obtained by transmission electron microscopyshows the existence of two-dimensional ordering associated with thesheets of hexagonal BN, with the appearance of broadened bands (10) and(11) and diffraction of the [100] and [110] rows of atoms (FIG. 5). Whenprepared, the molecules adopt an orientation parallel to the substrate(carbon film), making it impossible to reveal the diffraction lines ofthe (001) planes. However, the broadening of the hk diffraction bandsenables the size of these diffracting domains to be calculated(extension of sheets). Thus it was possible to measure L_(a)=1.2 nm,which could correspond to structures containing in the order of 15 to 20condensed rings.

After several days, the preparation changed form (formation of bubbles).

The solidification of the liquid crystal enables observations to be madewhich were inaccessible with the fresh preparation (unstable under thebeam).

The dark field observations (FIG. 6) on the walls of the bubbles make itpossible to contrast numerous small basic structural units (BSU)belonging to one and the same isoclinic domain. Lattice fringe imagingallows these coherent domains to be visualized. They consist of stacksof large molecules, as shown in the insert of FIG. 6.

By Energy Loss (EELS)

The same preparations as above were characterized by energy loss of theelectrons in the electron microscope. This technique makes it possibleto reveal (and quantify) the elements present. Furthermore, the finestructures at the thresholds of the absorption peaks (ELNES: ElectronLoss Near Edge Structure) make it possible to differentiate between thebonds (e.g. cubic BN or hexagonal BN).

FIG. 7 shows the spectrum obtained for the mesophase from which part ofthe solvent has been removed (ionic vacuum of the microscope): the Kthresholds of boron and nitrogen are visible at 188 eV and 401 eVrespectively. The fine structures present at the thresholds arecharacteristic of hexagonal BN. This was demonstrated by comparing saidspectrum on the one hand with that of a commercial cubic BN (supplied byDE BEERS) and on the other hand with that of a commercial BN-hex(supplied by PROLABO). These fine structures are due to the differentpossible transitions of the electrons. They are characteristic of thechemical environment of the atoms. Using this analysis, it is thuspossible to quantify the relative concentrations of the elements. Thus,in the case of a freshly prepared product, the NIB ratio isapproximately 1. When the polymer has been partially hydrolyzed, thestructures of the B and N lines broaden. Also, the appearance of a broadpeak is noted at E=532 eV, due to the presence of oxygen.

At low energies, the plasmon peak is observed; this corresponds to thecollective excitation of the valence electrons (FIG. 8). Between theloss-free peak and the plasmon peak, a second peak is observed; thiscorresponds to the interband excitations at E=8 eV. These transitions,identified as π→π*, are characteristic of the aromaticity of theproduct.

EXAMPLE 1′ (comparative) Synthesis of the Polyborazylene

Said synthesis is carried out under the conditions specified in Example1.

Ageing of said Polyborazylene

The polymer obtained is stored at around θ=0° C. for about 8 months.

After this low temperature storage, an attempt was made to characterizethe “mesophase” obtained.

Characterization of the Mesophase by Light Microscopy

Even after solubilization, the mesophase obtained from borazyleneaccording to the prior art does not lead to the same type of mesophaseas those of the invention. This is clearly revealed on the negative ofFIG. 2, where small anisotropic crystallites are observed. The slowcondensation reactions have allowed the anisotropic domains to grow andsaid small insoluble crystallites to form. Furthermore, the most highlycondensed polymers tend to form monocrystals, which are insoluble evenat high dilution factors.

EXAMPLE 2 Synthesis of the Polyborazylene

The dehydrogenation/condensation reaction is carried out under the sameconditions as those of Example 1 except that the initial dinitrogenpressure is 60.10⁵ Pa ( P_(N) ₂ o=60.10⁵ Pa).

Preparation of the Mesophase

An amount m=110 mg of polyborazylene is mixed with an amount m=100 mg ofborazine. The milky solution obtained is very fluid.

Characterization of said Mesoihase by Light Microscopy

As within the framework of Example 1 (and FIG. 1), light microscopy (cf.FIG. 3) shows interference colors, proving the formation of a liquidcrystal. In this case, it is noted that the mesophase obtained possesseslarger anisotropic domains, although the contours remain irregular.

EXAMPLE 3 Synthesis of the Polyborazylene

The polyborazylene is prepared under the conditions of Example 2.

Preparation of the Mesophase

An amount m=100 mg of polyborazylene is mixed with an amount m=90 mg oftoluene. Said toluene has first been dried over sodium or by azeotropicdistillation and then distilled over sodium. The milky solution obtainedis very fluid.

Characterization of said Mesophase

The mesophase obtained (milky solution) has a degree of anisotropygreater than 80% in the polarized light microscope. Light microscopy(FIG. 4) in this case shows broad anisotropic domains with more regularcontours.

EXAMPLE 4

The polyborazylene and the mesophase are obtained successively under theconditions of Example 2. Said mesophase is used to coat a carbon bundleby the dipcoating technique to form a coating which can act as aninterphase in a composite, for example with a carbon reinforcement andceramic matrix.

After coating, the deposit is treated at a temperature of about 60° C.for 24 hours in order to render it infusible. This treatment is followedby pyrolysis at 1000° C. in an inert atmosphere (argon) for 12 hours.

The mesophase exhibits perfect film-forming behavior. The film obtainedis in fact homogeneous over its thickness and over the whole length ofthe coated bundle. This is clearly apparent when considering thescanning electron microscope negative (of one of the filaments of thebundle) in FIG. 9.

EXAMPLE 5

The polyborazylene and the mesophase are obtained successively under theconditions of Example 2. The mesophase is used to coat a bundle ofsilicon carbide fibers by the dipcoating technique to form a coatingcapable of acting as an interphase in a composite.

The deposit is then treated at a temperature of about 60° C. for 24hours in order to render it infusible. This treatment is followed bypyrolysis at 1000° C. in an inert atmosphere (argon) for 12 hours.Silicon carbide is then deposited by chemical vapor phase depositionfrom a gaseous precursor consisting of a mixture ofmethyltrichlorosilane (MTS) and hydrogen (H₂) in a molar ratio of${\alpha = {\frac{H_{2}}{MTS} = 6}},$

at a temperature of 950° C. and under a pressure of 3 kPa, to give aunidirectional minicomposite with a fiber volume fraction of 40%. Thematerial is tensiletested and characterized in structural andmicrostructural terms.

The material, comprising an interphase derived from the mesophasicpolyborazylene precursor of the invention, has nonfragile behavior and,in particular, a rupture pattern characterized by substantial fiberpullout. This behavior is made possible by the presence of theinterphase, which acts as a deflector of cracks in the matrix. Saidinterphase, consisting of the material derived from the mesophasicpolyborazylene, forms the basis of the good mechanical behavior of thecomposite.

EXAMPLE 6

The polyborazylene and the mesophase are obtained successively under theconditions of Example 2. The mesophase is used as a precursor to boronnitride fibers. In this case, the amount of solvent used to obtain themesomorphic state is adjusted to give the appropriate viscosity. Whenthe mesophase reaches the desired viscosity, dry spinning is carried outat a temperature in the order of 70° C.

Analysis of the crude fibers by the EELS (energy loss) technique showsthat their composition is very similar to stoichiometry: B/N (atomic)=1.

The fibers are then heatstabilized. Finally, they are pyrolyzedsuccessively at 1000° C. for 12 hours and then at 1250° C. for 2 hoursunder argon. The transmission electron microscope negative shows theextent to which the addition of solvent does indeed make it possible toadjust the viscosity, since fiber diameters of less than 3 μm wereeasily obtained. The structure and chemical composition of the fibersconfirm the formation of a virtually stoichiometric boron nitride. At1250° C. this boron nitride already exhibits an advanced state of“graphitization”. Quantitative analysis of the electron diffractionnegatives shows the development of three-dimensional ordering. The boronnitride crystallites consist of about ten layers spaced apart by anaverage distance in the order of 0.336 nm (FIG. 10).

What is claimed is:
 1. A process for the preparation of a mesophase ofpolyborazylene, comprising the steps of: a. preparing polyborazylene bythe polycondensation of borazine in a closed reactor, and b. adding asolvent to said polyborazylene obtained by said polycondensation,resulting in the generation of said mesophase, wherein said solvent isselected from the group consisting of aromatic solvents, borazinesolvents and mixtures thereof.
 2. The process according to claim 1,characterized in that said mesophase is subsequently treated so as toremove all or part of said solvent therefrom.
 3. The process accordingto claim 1, characterized in that said polycondensation is carried outat a temperature of between 50 and 120° C.
 4. The process according toclaim 1, characterized in that said polycondensation is carried outunder an initial inert gas pressure of between atmospheric pressure and200.10⁵ Pa.
 5. The process according to claim 1, characterized in thatsaid solvent is selected from borazine, benzene, toluene and xylenes. 6.The process according to claim 1, characterized in that said solvent isadded in a solvent/polyborazylene weight ratio below
 10. 7. The processaccording to claim 6, characterized in that said solvent is added in asolvent/polyborazylene weight ratio greater than
 1. 8. The processaccording to claim 6, characterized in that said solvent is added in asolvent/polyborazylene weight ratio less than or equal to
 1. 9. Theprocess according to claim 3, characterized in that saidpolycondensation is carried out at a temperature of between 60 and 80°C.
 10. The process according to claim 6, characterized in that saidsolvent is added in a solvent/polyborazylene ratio between 0.2 and 5.11. Mesophase of polyborazylene in the presence of a solvent selectedfrom aromatic solvents, borazine solvents and mixtures thereof. 12.Mesophase of polyborazylene having a degree of anisotropy greater than50% in light microscopy.
 13. The mesophase of polyborazylene of claim12, wherein said degree of anisotropy is approximately 100% in lightmicroscopy.
 14. The mesophase of polyborazylene of claim 12 wherein saiddegree of anistropy is greater than 80% in light microscopy.
 15. Aprocess for preparing boron nitride from a precursor of boron nitride,comprising thermally treating said precursor, wherein said precursor isa mesophase of polyborazylene in the presence of solvent according toclaim
 11. 16. The process according to claim 15 for preparing hexagonalboron nitride.
 17. A process for preparing boron nitride from aprecursor of boron nitride, comprising thermally treating saidprecursor, wherein said precursor is a mesophase of polyborazyleneaccording to claim
 12. 18. The process according to claim 17 forpreparing hexagonal boron nitride.
 19. A process for preparing boronnitride from a precursor of boron nitride, comprising thermally treatingsaid precursor, wherein said precursor is a mesophase of polyborazyleneprepared according to claim
 1. 20. The process according to claim 19 forpreparing hexagonal boron nitride.